Preheat the air fryer to 390°F (199°C) if the manufacturer recommends preheating.
In a large bowl, whisk the gochujang, 40 ml hot water and Splenda until smooth. Add the remaining 10 ml water only if needed to make a thick, pourable marinade.
Pat the chicken dry, add it to the bowl and turn until every piece is evenly coated.
Lightly coat the air-fryer basket with cooking spray. Arrange the thighs in a single layer without overlapping; cook in batches as needed. Lightly spray the tops and season evenly with black pepper.
Air fry each batch for 10–12 minutes, turning halfway through, until browned and the thickest part of every thigh reaches at least 165°F (74°C). Air fryers and thigh thickness vary, so use temperature rather than crispness alone to determine doneness.
Rest for 3 minutes, then slice or serve whole with rice, a wrap or salad if desired.